Hard to say if it uses my data. It doesn't appear to but the data transfer is low, and they only show balances to the nearest 10MB on their website... Didn't seem like my data was used on receipt of a call.
I tested it, and it only depletes your voice minutes, and not your data. Used about 30 minutes on a call while connected to their 4G network. 20MB data transferred during the call, none of it deducted from my data balance.
Actually, that was on a call initiated from the rain sim.
Will see if...
TL : DR -> Don't set raintalk as your default dialler. Only then will it start working, and you'll be able to make and receive calls with it, including via wifi connection. Tested on a Huawei P9 Lite, EMUI 5.0.4, Android 7.0.
Greetings
So I have a 6-year-old Huawei P9 Lite, running Android...